1. Field of the Invention
The present invention relates to devices for controlling coastal erosion and more particularly relates to an improved matrix of truncated cone-like hollowed elements and connecting wave blocks which control wave action and simultaneously build up accretions by collecting solid material within the matrix and adjacent thereto.
2. General Background
Erosion, unlike the effects of hurricanes, is the simple loss of one grain of sand, one grain at a time.
Beaches, the most common shoreforms in the United States, are gentle slopes covered with loose sediment. The sediment particles ranging from fine silt to coarse gravel or cobbles in size, are moved by wind and water.
In calm weather, waves at the beach are usually low, long swells. These waves have less energy than choppy storm waves and do not cause as much turbulence when they break. Swells break and run up over the foreshore of the beach until they use up their energy. Then they drop back under the force of gravity. They tend to deposit material on the beach up to the normal high water line. At the high water line, a low ridge or "berm" may be formed by this type of wave action. During storms, water overtops the berm crest and washes over the backshore. The backshore area can be bordered on the inland side by dunes or the like, which are formed by the wind blowing sand and soil along the beach until it meets an obstruction.
The parts of the shore that extend into the water are more vigorously attacked than the shoreline of inlets or bays. Incoming waves tend to bend around these peninsulas, headlands, extended beaches, or seawalls, and concentrate their energy on the front and sides of the area. Extra protection or reinforcement is often needed on these exposed parts of the coast. In some coastal regions of the United States, disappearance of coastal wetlands is occurring at an alarming rate due to wave and wind action, saltwater intrusion, and settlement.
Wave motion, particularly that of breaking waves, is the most important active agent in the building and erosion of the shoreline. The characteristics of waves depend on the speed of the wind, its duration, and the unobstructed water distance, or "fetch," it blows over. As the waves break, run up the shore, and return they carry sedimentary material onshore and offshore. This sedimentary material is called littoral drift. Most waves arrive at an angle to the shore and set up a longshore current, moving littoral drift in a series of zigzags as successive wave fronts advance and retreat. The predominant direction of longshore transport is referred to as "downdrift"; the opposite direction is "updrift."
The ability of water to move material depends on its speed. Large waves or fast-moving currents can carry larger quantities and heavier littoral drift. Material picked up from inland heights, from river beds and banks, and from shoreline areas is deposited wherever the water is slowed down, and it may be picked up again when the velocity of water increases. Growing shores can be fed, or "nourished," by material that has been eroded from somewhere else. Often attempts to reduce erosion and build up one area will result in reduced deposition elsewhere, "starving" another shoreline. Erosion and accretion are two faces of the same process, which may either occur at extremely slow rates or make dramatic changes in the shoreline within a human lifetime.
Water level also influences the erosion process. Changes in high and low water levels due to seasons, tides, storms, droughts, or floods can expose new surfaces to erosion.
Seasons and storms, which affect the movement and level of water and the strength and direction of wind, alter patterns of erosion and deposition. Storms whip the water into waves higher than normal, resulting in rapid erosion of vulnerable areas and propelling stones or other debris onto shore with unusual force. As seasons turn, wind strength and direction also change, altering the path of waves and currents and resulting in new areas of erosion or accretion. Where ice forms, it reduces wave action, which may slow erosion, and at the same time it exerts tremendous horizontal and vertical forces that may weaken structures on the shore. Winter freeze and spring thaw affect rivers, streams, and lakes, changing their water levels and the speed of currents.
Although erosion can be caused by natural shoreline processes, its rate and severity can be intensified by human activity. The shoreline and the water are highly valued for recreational activities, but such may accelerate erosion. Those who build "permanent" homes and recreation facilities often ignore the fact that the shoreline is being constantly built up and worn away again. They may also fail to take into account the periodic and unpredictable effects of storms. Dredging for marinas and bulldozing of dunes for improved seascape views remove natural protection against wind and waves. Pedestrian and vehicular traffic also contribute to the destruction of shoreline defenses by destroying vegetation, degrading dunes, and weakening bluffs and banks. Docks, jetties, and other structures interrupt the natural shoreline movement of water and redirect erosive forces in unexpected and possibly undesirable directions. Saltwater intrusion into freshwater marshes can result when man digs navigation canals in the freshwater marsh. These canals become "speedways" for saltwater moving with the tides. The salt kills protective vegetation and erosion can be exponential as is presently occurring in Louisiana which has about forty percent (40%) of the nations's valueable wetlands, but the acreage is disappearing at a rate of hundred of acres per day. The loss of these wetlands is a well documented national problem that has long plagued the art.
The erosion problem is compounded by the removal of forests, overgrazing of land, burning, construction of highways and the like, and the channelization of streams.
Planting rapidly growing vegetation on areas which have been stripped is sometimes successful but usually only where the vegetation grows rapidly and extensively enough so that the soil is protected. In areas where water is continually flowing such as on riverbanks, the growth of vegetation can usually not be done quickly enough to prevent erosion. Various systems of revetment have been used to augment or replace vegetation as an erosion barrier. The art has use loose fill barriers (riprap), continuous paving mats (some with weep holes to relieve pore pressure) and porous paving mats to control erosion.
"Riprap" which is known for control of erosion is basically a barrier or coverage comprising a plurality of large chunks of concrete (obtained, for example, in salvage operations) which are dumped in a particular area. The concrete chunks are usually of random size, with some so large as to not provide protection and washouts occur underneath. Further, the placement is often random, not adequately covering the subject area.
Solid continuous paving mats of concrete are highly costly because of the extensive amount of concrete required, the difficulty and costs of installation, and the problems of hydrostatic pore pressure which are created once the concrete is in place.
Paving blocks of concrete mats and other materials are known. Flexible porous concrete mats have been used as an erosion controlling protective surface.
Different patents directed to using revetment blocks and structures for preventing soil erosion have been issued.
U.S. Pat. No. 4,227,820 discloses a device comprising a matrix of cellular concrete blocks, each of which has internal passageways for cables to pass therethrough and interconnect a matrix of concrete blocks. The free ends of the cables are anchored into the ground after which operation the soil is spread over the blocks to reinforce surface thus controlling soil erosion.
U.S. Pat. No. 4,152,875 discloses a ground covering with adjoining plates which are clamped together by tensioning elements extending through the plates and parallel to them.
Nijdorn in U.S. Pat. No. 3,922,865 describes a mattress having a filter cloth with metal bars woven thereinto. Spaced concrete blocks are connected to these bars.
Appelton in U.S. Pat. No. 3,903,702 describes the use of a revetment structure with similar interfitting units which form a flexible mattress. The units are provided with a series of interconnected ribs which make opposite sides of the units reflections of each other.
U.S. Pat. No. 3,597,928 discloses the use of porous flexible supporting sheets with mat of blocks which are placed on these sheets. Each mat consists of a plurality of blocks with drainage passageways therethrough and the blocks are secured to the sheets by adhesive means.
Nelson in U.S. Pat. No. 3,386,252 discloses a riprap structure for waterways, comprising rectangular blocks interconnected by a rod which extends through the blocks to provide for hooking the blocks at diagonally opposite corner ends and forming a matrix.
Dixon, U.S. Pat. No. 2,876,628 discloses a rapidly sinking articulated revetment for riverbanks comprising rigid blocks interconnected by flexible cables. The upper surface of each block has recesses from which openings extend through the whole block to provide for water passageways.
Louckes in U.S. Pat. No. 2,674,856 teaches the use of a similar flexible revetment mat which flexibility comes from the use of reinforced wires extending continuously from ne concrete block to another to form a mattress for protection of river banks from erosion.
U.S. Pat. No. 2,159,685 describes a concrete riprap consisting of precast units connected by interlocking bars which pass though the orifices in the body of each unit.
A revetment in U.S. Pat. No. 2,008,866 comprises a number of rectangular concrete blocks arranged diagonally and hooked together by crossed rods to form a mat.
U.S. Pat. No. 1,987,150 teaches the use of a revetment containing filled asphalt in a certain proportion. A mat of such asphalt is placed adjacent to a mattress consisting of slabs interlocked by cables or clips passed through the rings at each corner of slab.
U.S. Pat. No. 1,359,475 describes a seawall construction comprising concrete panels with mating tongues and grooves at their edges and locked together by metal rods passing through the notches in the tongues and grooves.
Edinger's U.S. Pat. No. 1,164,708 discloses an embankment protection construction composed of interlocking rectangular concrete slabs with integrally made hook flanges and interengaging keys and sockets for locking the slabs in a mattress.
Edinger's U.S. Pat. No. 1,164,707 discloses a flexible concrete slab revetment construction composed of concrete slabs with integrally formed concrete joints interlocking the slabs, these slabs being preferably of a triangular contour.
U.S. Pat. No. 763,171 teaches the use of embankment linings consisting of brick or stone blocks interlocked by wires passing through the perforations in block bodies.
Villa in U.S. Pat. No. 554,354 discloses a covering for protecting banks from erosion, this covering comprising cement or terra-cotta prismatie plates interconnected by wires which pass through the plates to form rows of units adapted to cover riverbeds and banks, and free ends of wires are fastened to trees or piles driven into the bank.
Flexible mats, though generally more expensive than riprap or continuous paving barriers, are usually more stable. Flexible mats are not as prone to under-cutting erosion, by water, and provide greater relief for hydrostatic pressure. Flexible mats do exhibit failure, however, when individual elements of the mat are displaced by hydrostatic pressure or wave action, for example.
In Barnett, U.S. Pat. No. 4,318,642, a method of making retaining walls utilizes stacked cylindrical and truncated conical concrete elements to be filled with soil or crushed rock. This method of stacking the conical elements leaves large gaps between the elements which must be "spanned" between by the 45 degree angle of repose of the crushed rock, or, by crushed rock larger than the annular gaps between stacked rings as shown in FIG. 20 and described as item "35". Any gap in any retaining wall subject to wave action will cause "leaching-out" of the material, day by day, by the incessant pounding of the waves.
In Fort, U.S. Pat. No. 2,653,450, a similar retaining wall structure of longer cylindrical elements which are heavier and more stable than the Barnett Patent is shown. The sheer weight of this wall requires a separate cast-in-place foundation which was not indicated in the Barnett Patent. In most cases of mud-slides and lost embankments due to poor soil, a very serious settlement and lateral movement of such a heavily constructed wall.
In Fort, U.S. Pat. No. 2,292,3340, a disclosure to provide better interlocking of the previous patent is shown. The weight of the units appears significant.
In Schlueter, U.S. Pat No. 1,893,303, a massive sea wall is described with man-sized elements which are apparently installed on a rock bottom by virtue of the pins "29" shown to anchor the interlocking hollow cylindrical tubular constructions to the sea-floor. The stability of the structure would be questionable on any seabed other than rock or coral, which, of course, do not have erosion problems.
In Pey, U.S. Pat. No. 4,083,190, a "breakwater" is constructed by stacked triangular elements. These units, if wide in horizontal dimension and shallow in vertical dimension may serve as substantial "breakwater" diffusers installed some distance from the shoreline. However the downward, vertical scouring action imposed by the vertical planes of these units would eventually cause severe tilting of the breakwater wall until it was toppled by strong seas. In any kind of vertical bulkhead or breakwater, the water is forced both upward and downward when the force of the wave contacts the wall. The downward force causes eddy currents which will undermine any vertical sea-wall with a shallow, articulated footing.
In Matsui, U.S. Pat. No. 4,225,269, similar structural foundation problems are obvious. This wall will eventually tilt backward against the wave or against the earth it is intended to restrain. The calculations of this effect is well known in Civil Engineering Practice. It may be obvious that the proposed design would be far more stable if utilized in the form shown, but inverted. The need for the "props" 32 illustrate this problem with the top-heavy inverted cones.
In Frohwerk, U.S. Pat. No. 4,481,155, a series of egg-crate tile boxes are stacked for cooling tower purposes.
In Atkinson, U.S. Pat. No. 4,372,705, an articulated concrete mat of interlocking concrete elements are intended to stabilize a sea bottom or beach. The geometry allows for vertical differential settlement or placement on an uneven bottom. The number of units to effect a solution is substantial and therefore must be installed as a very wide mat for good effect.